This invention relates to an apparatus for traversing a flat surface having a multiplicity of openings, and in particular to an apparatus for traversing the tube sheet of a nuclear steam generator.
The servicing of steam generators continues to be of major importance in the operation of nuclear plants, particularly those having a pressurized water nuclear steam supply system. These steam generators are in effect large heat exchangers between the primary coolant loop, which is somewhat radioactive, and the secondary, or steam loop, which must be maintained at very low levels of radioactivity. For a variety of reasons, a number of the steam generator tubes deteriorate with time and must be either sleeved or plugged to prevent primary coolant from entering the secondary loop. This and other steam generator maintenance activities are performed during outages when the steam generator can be drained of fluid. Nevertheless, residual radioactivity levels in the steam generator are too high for humans to tolerate safely for the duration of time necessary to make the repairs manually. As a result, the prior art includes a number of remotely-controlled devices, commonly called "finger walkers", for performing various maintenance functions without direct human presence. In particular, such finger walkers are adapted to selectively engage and disengage from the openings in the tube sheet whereby the finger walker may be advanced in one of several directions along the tube sheet. The finger walker typically carries tools with which the tube maintenance or repair operations can be remotely performed.
U.S. Pat. Nos. 3,889,820; 3,913,452; and 3,913,752 are directed to a finger walker having perpendicularly orientated bases which carry fingers for selective insertion into and withdrawal from the tube sheet openings. In this device, the bases move perpendicularly relative to each other in a plane parallel to the tube sheet surface, and the fingers are selectively moveable perpendicularly to the tube sheet and bases.
U.S. Pat. Nos. 4,018,344; 4,018,345; and 4,018,346 are directed to an improvement in the previously described finger walker, wherein the perpendicularly oriented bases are adapted to move relative to each other in a direction perpendicular to the tube sheet, as well as moving relative to each other in a plane parallel to the tube sheet.
U.S. Pat. No. 4,074,814 is directed to a method of sequentially actuating the bases and fingers of the above-type finger walkers.
A more recent improvement to both of the previously described finger walkers is disclosed in U.S. Pat. No. 4,193,735. This improvement provides a rotating platform attached to the finger walker such that a plurality of tools, in sequence, can be deployed at a common area of the tube sheet. In this apparatus, the base members are perpendicularly oriented for moving along "X-Y" incremental directions, with the "X" base and fingers thereon adapted to move also in the direction perpendicular to the tube sheet, whereas the "Y" base equivalent remains in a fixed plane parallel to the tube sheet with the fingers carried thereon being moveable toward and away from the tube sheet.
With all the foregoing prior art finger walkers, advancement of the apparatus along the tube sheet is achieved by sequential movements in either the "X" or "Y" direction by the respective, perpendicularly oriented "X" and "Y" bases.
Although the prior art finger walkers described above have performed satisfactorily in actual field use, their complexity, when coupled with the tendency of tolerances to accumulate as the finger walker traverses the tube sheet, results in occasional failure to advance as required. Human intervention is then necessary to manually relocate the finger walker. Also, the perpendicular orientation of the "X" and "Y" arms is not ideal for traversing a steam generator tube sheet having the tubes spaced on a triangular, rather than square, array. Furthermore, because the fingers carried by the "X-Y" base members are relatively close together, they sometimes loosen when tools carried by the platform or other extension of the finger walker generate unbalanced forces on the fingers when deployed. Thus, the prior art sometimes includes stabilizing fingers on the platform, having no locomotive function whatsoever.